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Register a new userRevisiting the Stellar Velocity Ellipsoid - Hubble type relation: observations versus simulations
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The stellar velocity ellipsoid (SVE) in galaxies can provide important information on the processes that participate in the dynamical heating of their disc components (e.g. giant molecular clouds, mergers, spiral density waves, bars). Earlier findings suggested a strong relation between the shape of the disc SVE and Hubble type, with later-type galaxies displaying more anisotropic ellipsoids and early-types being more isotropic. In this paper, we revisit the strength of this relation using an exhaustive compilation of observational results from the literature on this issue. We find no clear correlation between the shape of the disc SVE and morphological type, and show that galaxies with the same Hubble type display a wide range of vertical-to-radial velocity dispersion ratios. The points are distributed around a mean value and scatter of $sigma_z/sigma_R=0.7pm 0.2$. With the aid of numerical simulations, we argue that different mechanisms might influence the shape of the SVE in the same manner and that the same process (e.g. mergers) does not have the same impact in all the galaxies. The complexity of the observational picture is confirmed by these simulations, which suggest that the vertical-to-radial axis ratio of the SVE is not a good indicator of the main source of disc heating. Our analysis of those simulations also indicates that the observed shape of the disc SVE may be affected by several processes simultaneously and that the signatures of some of them (e.g. mergers) fade over time.
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The scatter in the galaxy size versus stellar mass (Mstar) relation gets largely reduced when, rather than the half-mass radius Re, the size at a fixed surface density is used. Here we address why this happens. We show how a reduction is to be expected because any two galaxies with the same Mstar have at least one radius with identical surface density, where the galaxies have identical size. However, the reason why the scatter is reduced to the observed level is not trivial, and we pin it down to the galaxy surface density profiles approximately following Sersic profiles with their Re and Sersic index (n) anti-correlated (i.e., given Mstar, n increases when Re decreases). Our analytical results describe very well the behavior of the observed galaxies as portrayed in the NASA Sloan Atlas (NSA), which contains more than half a million local objects with 7 < log(Mstar/Msun) < 11.5. The comparison with NSA galaxies also allows us to find the optimal values for the mass surface density (2.4m0.9p1.3 Msun/pc2) and surface brightness (r-band 24.7pm0.5 mag/arcsec2) that minimize the scatter, although the actual values depend somehow on the subset of NSA galaxies used for optimization. The physical reason for the existence of optimal values is unknown but, as Trujillo+20 point out, they are close to the gas surface density threshold to form stars and thus may trace the physical end of a galaxy. Our NSA-based size--mass relation agrees with theirs on the slope as well as on the magnitude of the scatter. As a by-product of the narrowness of the size--mass relation (only 0.06 dex), we propose to use the size of a galaxy to measure its stellar mass. In terms of observing time, it is not more demanding than the usual photometric techniques and may present practical advantages in particular cases.
We report a tight linear relation between the HI circular velocity measured at 6 $R_{rm e}$ and the stellar velocity dispersion measured within 1 $R_{rm e}$ for a sample of 16 early-type galaxies with stellar mass between $10^{10}$ and $10^{11}$ $mathrm{M}_odot$. The key difference from previous studies is that we only use spatially resolved $v_mathrm{circ}$(HI) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that $v_mathrm{circ}$(HI) = 1.33 $sigma_mathrm{e}$ with an observed scatter of just 12 percent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The $v_mathrm{circ}$(HI)-$sigma_mathrm{e}$ relation is shallower than those based on $v_mathrm{circ}$ measurements obtained from stellar kinematics and modelling at smaller radius, implying that vcirc declines with radius -- as in bulge-dominated spirals. Indeed, the value of $v_mathrm{circ}$(HI) is typically 25 percent lower than the maximum $v_mathrm{circ}$ derived at $sim0.2 R_mathrm{e}$ from dynamical models. Under the assumption of power-law total density profiles $rho propto r^{-gamma}$, our data imply an average logarithmic slope $langlegammarangle=2.18pm0.03$ across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.
We present the relation between stellar specific angular momentum $j_*$, stellar mass $M_*$, and bulge-to-total light ratio $beta$ for THINGS, CALIFA and Romanowsky & Fall datasets, exploring the existence of a fundamental plane between these parameters as first suggested by Obreschkow & Glazebrook. Our best-fit $M_*-j_*$ relation yields a slope of $alpha = 1.03 pm 0.11$ with a trivariate fit including $beta$. When ignoring the effect of $beta$, the exponent $alpha = 0.56 pm 0.06$ is consistent with $alpha = 2/3$ predicted for dark matter halos. There is a linear $beta - j_*/M_*$ relation for $beta lesssim 0.4$, exhibiting a general trend of increasing $beta$ with decreasing $j_*/M_*$. Galaxies with $beta gtrsim 0.4$ have higher $j_*$ than predicted by the relation. Pseudobulge galaxies have preferentially lower $beta$ for a given $j_*/M_*$ than galaxies that contain classical bulges. Pseudobulge galaxies follow a well-defined track in $beta - j_*/M_*$ space, consistent with Obreschkow & Glazebrook, while galaxies with classical bulges do not. These results are consistent with the hypothesis that while growth in either bulge type is linked to a decrease in $j_*/M_*$, the mechanisms that build pseudobulges seem to be less efficient at increasing bulge mass per decrease in specific angular momentum than those that build classical bulges.
The Halo Assembly in Lambda-CDM: Observations in 7 Dimensions (HALO7D) dataset consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured proper motions of Milky Way (MW) halo main sequence turnoff stars in the CANDELS fields. In this paper, the second in the HALO7D series, we present the proper motions for the HALO7D sample. We discuss our measurement methodology, which makes use of a Bayesian mixture modeling approach for creating the stationary reference frame of distant galaxies. Using the 3D kinematic HALO7D sample, we estimate the parameters of the halo velocity ellipsoid, $langle v_{phi} rangle, sigma_r, sigma_{phi}, sigma_{theta}$, and the velocity anisotropy $beta$. Using the full HALO7D sample, we find $beta=0.63 pm 0.05$ at $langle r rangle =24$ kpc. We also estimate the ellipsoid parameters for our sample split into three apparent magnitude bins; the posterior medians for these estimates of $beta$, while consistent with one another, increase as a function of mean sample distance. Finally, we estimate $beta$ in each of the individual HALO7D fields. We find that the velocity anisotropy $beta$ can vary from field to field, which suggests that the halo is not phase mixed at $langle r rangle =24$ kpc. We explore the $beta$ variation across the skies of two stellar halos from the textit{Latte} suite of FIRE-2 simulations, finding that both simulated galaxies show $beta$ variation over a similar range to the variation observed across the four HALO7D fields. The accretion histories of the two simulated galaxies result in different $beta$ variation patterns; spatially mapping $beta$ is thus a way forward in characterizing the accretion history of the Galaxy.
The ellipsoid of stellar random motions is a fundamental ingredient of galaxy dynamics. Yet it has long been difficult to constrain this component in disks others than the Milky Way. This article presents the modeling of the azimuthal-to-radial axis ratio of the velocity ellipsoid of galactic disks from stellar dispersion maps using integral field spectroscopy data of the CALIFA survey. The measured azimuthal anisotropy is shown to be not strongly dependent on the assumed vertical-to-radial dispersion ratio of the ellipsoid. The anisotropy distribution shows a large diversity in the orbital structure of disk galaxies from tangential to radial stellar orbits. Globally, the orbits are isotropic in inner disk regions and become more radial as a function of radius, although this picture tends to depend on galaxy morphology and luminosity. The Milky Way orbital anisotropy profile measured from the Second Gaia Data Release is consistent with those of CALIFA galaxies. A new correlation is evidenced, linking the absolute magnitude or stellar mass of the disks to the azimuthal anisotropy. More luminous disks have more radial orbits and less luminous disks have isotropic and somewhat tangential orbits. This correlation is consistent with the picture in galaxy evolution in which orbits become more radial as the mass grows and is redistributed as a function of time. With the help of circular velocity curves, it is also shown that the epicycle theory fails to reproduce the diversity of the azimuthal anisotropy of stellar random motions, as it predicts only nearly radial orbits in the presence of flat curves. The origin of this conflict is yet to be identified. It also questions the validity of the vertical-to-radial axis ratio of the velocity ellipsoid derived by many studies in the framework of the epicyclic approximation.
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